A number of off-gas streams containing hydrogen and hydrocarbons are generated during refinery and petrochemical plant operations. These streams include overheads from: phase separators; fractionation columns; stabilization columns; demethanizers; debutanizers; absorption, stripping and scrubbing units; and so on. In some cases, the composition of the stream renders it suitable for reintroduction into the train of operations upstream or downstream of its generation point. Frequently, however, the stream composition is such that it is not cost-effective to treat it further and it is passed to the plant fuel header.
Streams passed to the fuel header typically contain a mixture of light hydrocarbons, heavier hydrocarbons and hydrogen. The heavier hydrocarbons represent lost product, or at least may have a higher value as LPG than as fuel gas. Most refineries currently operate with a hydrogen deficit, which would be reduced if more hydrogen could be recovered from ongoing operations. In addition, only a finite quantity of fuel gas is needed, so some plants are bottlenecked by over supply. In these bottleneck situations, reduction in the amount of fuel gas produced, and/or control of the Btu value of that gas by reducing the heavier hydrocarbon content, would enable throughput of the unit operations in the refinery train, such as hydrotreating or reforming, to be increased.
Techniques exist that, in principle, can remove hydrocarbons and hydrogen to essentially any desired degree. For example, C.sub.4+ and heavier hydrocarbons can be removed from the stream by cooling, compression, or a combination of both. Cooling is indeed often used, such as to treat the hot vapors that form the raw reactor effluent from hydroprocessing, aromatics manufacture and the like. Streams can be cooled by heat exchange against incoming fluids, by air cooling, water cooling or use of external refrigerants. Practical limits are set by availability and cost of coolants, however, and the lower the temperature, the harder the economic justification becomes. Absorption into lean oils can be used, but again, performance is limited by the pressure and temperature conditions under which the process is carried out, which in turn are controlled by cost factors.
For hydrogen recovery from light hydrocarbons, techniques that have been deployed in refineries and petrochemical plants include pressure swing adsorption (PSA) and membrane separation. Representative references that teach the use of PSA to treat off-gases from petrochemical processes include U.S. Pat. Nos. 5,332,492 and 5,457,256, to UOP, and U.S. Pat. No. 5,675,052, to BOC. The literature also contains numerous references to membrane separation processes for hydrogen/hydrocarbon separation in refineries. For example, U.S. Pat. Nos. 4,362,613 and 4,367,135, to Monsanto, describe processes for treating overhead vapors from phase separators in a hydrocracking plant. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes use of a membrane for removal of hydrogen from cracking effluent. U.S. Pat. No. 5,157,200, to Institute Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons. Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063, to Air Products, and 4,892,564, to Cooley.
The use of polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and "Optimizing Hydrocracker Hydrogen", by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners.degree. Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various membrane-based hydrogen separations.
In all of the above cases, the membranes used to perform the hydrogen/hydrocarbon separation are hydrogen-selective, that is, they permeate hydrogen preferentially over hydrocarbons and all other gases in the mix.
A difficulty that hampers the use of both PSA systems and membrane separation systems of the type described above is the presence in off-gases of the heavier hydrocarbons, water vapor and hydrogen sulfide. These materials cause a variety of problems. In the case of PSA systems, they may sorb preferentially onto the bed, both reducing the capacity of the beds to sorb the light hydrocarbons that they are intended to remove, and giving rise to serious regeneration difficulties, as discussed below.
In the case of membrane systems, the presence of these materials can cause catastrophic collapse of the membranes. For example, a report by N. N. Li et al. to the Department of Energy ("Membrane Separation Processes in the Petrochemical Industry", Phase II Final Report, September 1987) presents data showing the effect of water vapor on membrane flux for cellulose acetate membranes, and concludes that "for relative humidities of 30% and higher, the flux decline is large, rapid, and irreversible". E. W. Funk et al. ("Effect of Impurities on Cellulose Acetate Membrane Performance", Recent Advances in Separation Techniques--III, AlChE Symposium Series, 250, Vol 82, 1986) advocate that "Moisture levels up to 20% RH appear tolerable but higher levels can cause irreversible membrane compaction". Similar or worse problems can occur if liquid hydrocarbons are allowed to come into contact with membranes surfaces, as well as glues or other components used in the membrane modules. Although the feed gas to the inlet of the membrane separation system may be comfortably above its dewpoint, as the gas travels along the modules and is depleted in the faster permeating hydrogen, the hydrocarbon content of the residue can quickly build up, raising the dewpoint temperature sufficiently for hydrocarbon condensation in the modules to take place. To avoid this, either the gas must be heated at least 10.degree. C. above the highest dewpoint temperature that will be reached, or the more condensable hydrocarbons must be removed to a low level before the gas enters the membrane system.
To deal with these issues, both for PSA and membrane systems, pretreatment of hydrogen/light hydrocarbon mixtures must almost always be carried out, so that the PSA system or the membrane system is adequately protected and sees only a clean hydrogen/light hydrocarbon stream. A number of references show combinations of PSA with upstream treatment. For example, U.S. Pat. No. 5,675,052, to BOC, teaches treatment of off-gases from an alkylation process, in which the raw alkylate is compressed and cooled to condense almost all of the hydrocarbons in the stream, and the remaining hydrogen/light hydrocarbon mix is sent for PSA treatment. U.S. Pat. No. 5,457,256, to UOP, concerns treatment of dehydrogenation gases. The gases are first dehydrated to lower the water content to below 5 ppm. This highlights yet another difficulty, in that, if cooling below 0.degree. C. is used to remove hydrocarbons, the stream thus treated must first be dried to avoid ice formation in the subsequent hydrocarbon condensation step. After the gas has been dried, a cold box is used to reduce the gas temperature to between -29.degree. C. and -117.degree. C. The resulting uncondensed hydrogen/methane stream is finally sent for PSA treatment. A very similar scheme is described in U.S. Pat. No. 5,332,492, to UOP. In this case, the raw gas stream to be treated is typically from a catalytic reforming process. The process employs a PSA step preceded by a simple precooling step. Nevertheless, the simple precooling step requires the gas to be refrigerated between -9.degree. C. and -26.degree. C. The patent mentions that drying, such as with a glycol dessicant, must be used before the refrigeration step if the gas contains water vapor.
A reference that shows condensation to remove hydrocarbons upstream of a membrane separation step in a refinery is U.S. Pat. No. 5,452,581, to Dinh et al. Effluent from an ethylene manufacturing operation is cooled to a temperature below 0.degree. C., such as -30.degree. C. to -50.degree. C., before passing the remaining stream to a hydrogen-selective membrane. Interestingly, in this case, the membrane is specifically used to raise the dewpoint of the remaining stream to facilitate subsequent cryogenic condensation.
Besides individual treatment by PSA or membranes, numerous processes are known in which membrane separation (using conventional glassy, hydrogen-selective membranes) and PSA are combined in a complementary way to carry out an integrated process. These include the following U.S. Pat. No. 4,229,188, in which a guard absorber removes heavier hydrocarbons prior to a PSA/membrane hybrid separation; U.S. Pat. No. 4,238,204, in which the PSA unit precedes the membrane unit; U.S. Pat. No. 4,690,695, in which the membrane unit precedes the PSA unit; U.S. Pat. No. 4,701,187, in which a two-stage membrane unit is used in conjunction with a PSA unit; U.S. Pat. No. 4,783,203, in which the reject gas from the membrane separation step is used as displacement gas in the upstream PSA regeneration step; U.S. Pat. No. 4,836,833, in which PSA and membranes are used in either order to treat steam reformer off-gases after carbon dioxide removal; and U.S. Pat. No. 4,863,492, in which reject gases from a membrane separation step followed by a PSA step are combined to make a blended product. These numerous references all have two features in common. First, the membrane unit and the PSA unit are used in a complementary way to perform the same separation. Secondly, insofar as they relate to hydrogen/hydrocarbon separations, the membrane units all use hydrogen-selective membranes.
It is possible, however, to carry out separations in which hydrocarbons permeate selectively and hydrogen is rejected in the residue stream. Processes that rely on selective permeation of hydrocarbons to separate at least some hydrocarbons from at least some other less condensable gases are taught, for example, in U.S. Pat. Nos. 4,857,078; 4,963,165; 5,032,148; 5,089,033; 5,199,962, 5,281,255; 5,401,300; 5,407,466; 5,407,467; and 5,501,722, all to Membrane Technology and Research (MTR). In particular, U.S. Pat. No. 4,857,078, to Watler/MTR, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
An alternative approach using membranes that reject hydrogen and preferentially permeate hydrocarbons is to use not a polymeric membrane but a carbon membrane, such as those taught in U.S. Pat. No. 5,104,425, to Air Products and Chemicals. These membranes are made up of a microporous adsorbent material on a porous substrate, and can separate gas mixtures based on selective adsorption onto the pore walls, rather than by the solution/diffusion mechanism of conventional polymeric membranes. Thus, the mechanism of separation is akin to the separation mechanism in PSA. This allows separation between various hydrocarbon fractions to be made, and hydrogen tends to be retained in the membrane residue stream.
It is known to combine these membranes with PSA to carry out integrated separations of light hydrocarbons from hydrogen. U.S. Pat. No. 5,332,424 describes fractionation of a gas stream containing C.sub.1 -C.sub.4 hydrocarbons and hydrogen using a bank of membrane modules followed by a PSA unit. U.S. Pat. No. 5,354,547 teaches adsorbent carbon membranes followed by PSA for treating steam reformer off-gases. U.S. Pat. No. 5,435,836 teaches PSA followed by adsorbent carbon membranes for a similar separation, and U.S. Pat. No. 5,507,856 teaches a carbon membrane/PSA design for hydrocarbon/hydrogen separations in general, including sweeping of the permeate side of the membrane with reject gas from the PSA step. U.S. Pat. No. 5,634,354 teaches combinations of adsorbent membranes and PSA to treat gases containing hydrogen and olefins.
Adsorbent membranes systems similar to those disclosed in the above patents are described in papers by M. B Rao and S. Sirkar in Journal of Membrane Science (Vol. 85, 253-264 (1993)) and Gas Separation and Purification (Vol. 7, No. 4, 279-284 (1993)). Adsorbent membrane/PSA hybrid systems are described in some detail in reports by M. Anand and K. A. Ludwig to the U.S. Department of Energy ("Novel Selective Surface Flow Membranes for the Recovery of Hydrogen from Waste Gas Streams", Phase I (1995) and Phase II (1996) Final Reports under contract number DE-FC04-93AL94461), and in materials distributed at a U.S. Department of Energy, Office of Industrial Technology, exhibit in Washington, D.C. ("Scale-Up of Selective Surface Flow Membrane for Gas Separation", T. Nahieri et al., Air Products and Chemicals, 1996).
In all of the above references, the gas mixtures introduced into the adsorbent carbon membrane system are limited to those containing no heavier than C.sub.4 hydrocarbons. In fact, the references are explicit that a pretreatment system (temperature swing adsorption) is used to remove C.sub.5+ hydrocarbons, water vapor and hydrogen sulfide that might foul the membranes. Since the membranes rely on adsorption for their separation properties, they are vulnerable to the same problems as PSA systems, namely that the more readily is a component sorbed, the more difficult is it to desorb. These contaminants, once introduced into the membranes, block the sorption sites and prevent the membranes functioning for their intended purpose.